Method for detecting leaks in an above ground petroleum storage tank

ABSTRACT

In using the method for testing for leaks, in an above ground tank of liquid having a lower specific gravity and a lower electrical or thermal conductivity than water, first an amount of water sufficient to cover the bottom of the tank is introduced into the tank after which the level of the water in the tank is determined and changes in the level of the water in the tank over time are sensed. Additionally, the amount and rate of leakage relative to the capacity of the tank and the change in the level of the water of the tank over a selected time period are determined from the given parameters and the change in water level.

BACKGROUND OF THE INVENTION

The present invention relates to a method utilizing an electrical orthermal conductivity gauge to test for leaks in above ground storagetanks containing a liquid having a lower specific gravity and lowerelectrical or thermal conductivity than water in the tank.

Leaks in large above ground petroleum product storage tanks containingliquids such as regular gasoline, light fuel oil, diesel fuel andregular gasoline are a serious problem to the petrochemical industryfrom the standpoint of both inventory losses and environmentaldegradation.

The large size of such tanks makes leak detection difficult. In thisrespect, a leak rate of one barrel per hour in a 100 foot diameter tankwill produce a level change rate of approximately 0.01 inches per hourand a direct measurement of the level change at the surface of theliquid in the tank is difficult due to the resolution required and tothermal expansion of the liquid, the tank and the detecting apparatus.

Furthermore, acoustic emission methods of leak detection previouslyutilized provide results which are clouded by the weakness of the leakas an acoustic source and by signal attenuation in the liquid or in thetank shell. Furthermore, background noise from wind and thermalexpansion also appear to limit the reliability of leak detectiondeterminations utilizing acoustic emissions.

Experience has shown that all significant leaks in above ground tanksoccur in the area of the tank bottom. Also, experience has shown thatthermal conditions near the bottom of the tank are stable. Furthermore,surface waves and convection currents in the liquid are effectivelydampened by the time they reach the bottom of the tank.

With these observations in mind, the method of the present inventionutilizes the environment adjacent the bottom of the tank for detectingleaks. As will be described in greater detail hereinafter, the leakdetector utilized in practicing the method is adapted to be positionedin the stable zone adjacent the bottom of the tank.

SUMMARY OF THE INVENTION

The method of the present invention utilizes sensors which senseelectrical or thermal conductivity in the stored liquid and in waterintroduced into the tank adjacent the bottom of the tank. The sensorsand associated circuitry are not affected by outside temperature,pressure, etc. Furthermore, empirical tests indicate that the plane orinterface between the petrochemical liquid and an induced water layer atthe bottom of the tank is stable, except for a drop in the level of thewater layer due to a leak. On rare occasions, the level may increase dueto small and accountable amounts of water coming from condensation andrainfall.

According to the teachings of the present invention, there is provided amethod for determining the presence of a leak in an above ground tank ofliquid hydrocarbons having a lower specific gravity and a lowerelectrical or thermal conductivity than water, said method comprisingthe steps of: determining the level of water in said tank; sensingchanges in the level of water in the tank over time by sensing changesin conductivity at points along a line extending across an interfacebetween the liquid hydrocarbon and the water in the tank; and,determining the amount and rate of leakage relative to the capacity ofthe tank and the change in the level of the water in the tank over aselected time period.

In practicing the method, prior to making a test for leaks, water isintroduced into the tank to form a water layer on the bottom of thetank. Typically, this is accomplished the night before testing to allowequilibrium in the water layer and water-liquid interface to take place.

The leak detector includes a sensor assembly adjustably mounted in acasing. The casing typically has a vertical layer of water sensingmaterial thereon and is lowered into the tank and allowed to settle onthe bottom of the tank. The portion of the layer of sensing materialdisposed in the water will change color so as to establish a water levelindicating line.

The leak detector is lifted out of the tank and the position of thewater level line is noted. Then the sensor assembly comprising an arrayof bare uninsulated conductor ends or thermal sensors which are arrangedin a line, is positioned in the area of the water level with the line ofthe array extending at an angle to the horizontal and across orintersecting the plane containing the water level line.

The leak detector is then lowered back into the tank and placed on thebottom of the tank.

The wire conductors are insulated except for the end portions thereof sothat electrical conduction between the conductor end portions can onlytake place through the product liquid or through the water, or through acombination of both. The electrical resistance or thermal resistance ofthe product liquid is several orders of magnitude greater than that ofthe water layer which contains electrolytes. With respect to electricalresistance, typically the electrical resistance of the productapproaches infinite ohms and typically the electrical resistance of thewater is 8 to 20 megohms. Typically, the uninsulated wire end portionsforming electrical conductivity sensors are spaced approximately 0.05inches from each other. To obtain higher sensitivity or higherresolution, one can tilt the line of the array of sensors or have afixed line of the array of sensors located at an angle of inclination tothe horizontal or interface. Resolution is typically defined as L tan θinches where θ is the angle of the inclination of the sensor array lineand L is the center-to-center spacing between sensors.

The electrical conductivity can be measured by a standard ohm meter, aconductivity meter or a hand held digital volt-ohm meter. An a.c.conductivity meter is preferred to avoid polarization at the electrodesensors. In one preferred embodiment of a leak detector system utilizingthe leak detector of the present invention, a panel containing an arrayof lamps realized by light emitting diodes (LEDs) was utilized forindicating which sensors were in the product liquid and which were inthe water layer.

By correlating changes in the level of the water layer, as determined bychanges in the conductivity through the sensor profile with respect tothe size of the tank and the time period during which the change in thelevel occurred, such as, by means of a simple calculation or optionallywith a microprocessor, one can determine the leak rate.

A typical testing period will be from a half day to two days.

Old tanks which have been in use for some period of time may alreadycontain a bottom layer of water. A matted layer of debris can be formedat the interface containing emulsified material probably produced bybiological action. To ensure that this layer of stringy, mattedemulsified material does not clog up the sensors and adversely interferewith a measurement, the array of sensors of the sensor assembly of thedetector are preferably mounted within the casing and a structure isprovided for minimizing, if not altogether preventing, the interfacelayer of stringy matted emulsified material from gaining access to theinterior of the casing.

Additionally, in one preferred embodiment, a flushing system isincorporated into the detector for flushing or washing the sensors.

Typically, the above ground petrochemical storage tanks are of the typewhich have a bottom drainoff faucet coupled through a valve to a pipeextending from the bottom area of the tank. As a result, once a leaktest is completed the water introduced into the tank can be drained off.A known rate of drain-off can simulate a leak and verify the working ofthe leak detector and the results obtained can be used for calibratingthe leak detector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary vertical sectional view, with large portionsbroken away, of an above ground petroleum storage, tank showing the leakdetector of the present invention resting on the bottom of the tank.

FIG. 2 is an exploded perspective view of a cylindrical casing for theleak detector showing a sensor assembly separated from the cylindricalcasing for housing the sensor assembly.

FIG. 3 is a vertical sectional view through the leak detector shown inFIG. 1.

FIG. 4 is a top plan view of the leak detector shown in FIG. 3 and istaken along line 4--4 of FIG. 3.

FIG. 5 is a horizontal sectional view of the leak detector shown in FIG.3 and is taken along line 5--5 of FIG. 3.

FIG. 6 is a fragmentary vertical view of the leak detector shown in FIG.3 and is taken along line 6--6 of FIG. 3.

FIG. 7 is an enlarged vertical plan view with portions broken away of asensor array of the sensor assembly shown in FIGS. 2 and 3.

FIG. 8 is a vertical plan view of another embodiment of the sensor arrayshown in FIG. 7.

FIG. 9 is a block, partially mechanical partially schematic drawing, notto scale, of one embodiment of the overall detecting system includingthe leak detector, a control panel with lamps and conductivity meter andan optional microprocessor.

FIG. 10 is a schematic circuit diagram of an electrical circuit that ismounted in the control panel shown in FIG. 9 and is connected to onesensor conductor in the sensor assembly shown in FIGS. 7 or 8.

FIG. 11 is a fragmentary vertical sectional view of an above groundstorage tank, with major portions broken away, and shows a modifiedembodiment of the leak detector of the present invention resting on thebottom of the tank.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1 there is shown a fragmentary upper wall portion or top 8 of apetroleum storage tank 10 with an access hole 12 therein which isadapted to be sealed by a cap, and through which the leak detector 14 ofthe present invention can be inserted for being lowered to the bottom orbottom wall 16 of the tank 10 and then utilized for checking for leaksand subsequently withdrawn from the top 8 of the tank 10.

The leak detector 14 is shown resting on the bottom wall 16 of the tank10.

Typically, in a petroleum storage tank 10 a small amount of moisture orwater will accumulate on the bottom 16 of the tank 10, such as fromcondensation. However, in using the leak detector 14 a small amount ofwater is inserted into a tank 10 being tested for leaks sufficient tocover the bottom 16 of the tank 10. Also, experience has shown that ifthere is a leak in the tank 10, it usually is in the bottom wall 16 orin a lower end portion of the side wall (not shown) of the tank adjacentthe bottom wall 16. Accordingly, when there is a leak, level 18 of awater layer 20 will tend to fall.

The hydrocarbon, petroleum or other petrochemical liquid 22 stored inthe tank 10 typically has a low electrical conductivity (a resistivityof effectively infinite ohms - cm), while the water 20 at the bottom 16of the tank 10 will typically have a much higher conductivity (aresistance of 8-20 M Ω- cm) as a result of the electrolytes therein.Also, at an interface 24 between the hydrocarbon liquid 22 which issituated above the water layer 20, there is a transition layer 24 whichhas a varying conductivity and which under certain conditions may bemade up of a thin mat or layer 24 of debris that have somehow foundtheir way into the tank 10 or have been formed therein.

As will be described in greater detail hereinafter, the leak detector 14of the present invention enables one to accurately determine thepresence of a leak by determining changes in the water level 18, i.e.,changes in the height of the interface 24 above the bottom 16 of thetank 10. This is determined by detecting changes in thermal orelectrical conductivity or current flow in sensor circuits 30 (FIG. 10)for example coupled to a sensor assembly 32 (FIG. 2) located at theinterface 24 between the upper liquid hydrocarbon 22 and the lower waterlayer 20.

The leak detector 14 will now be described below with reference to asensor assembly 32 for sensing electrical conductivity after which athermal sensor assembly 322 for sensing thermal conductivity will bedescribed in connection with the description of FIG. 8.

As shown in FIG. 2, the leak detector 14 of the present inventioncomprises the sensor assembly 32 which is received through an upper openend 36 of a cylindrical casing 38 having mounted at the lower end 40thereof a base assembly 42. The base assembly 42 typically includes anannular ring member 44 having affixed therein a generally circular flatbottom plate or wall 46 (FIG. 3) which closes off the lower end 40 ofthe cylindrical casing 38. Mounted to the bottom wall 46 are three feet51, 52, 53 which extend downwardly and which include threadablyadjustably extendable, pointed prongs 61, 62 and 63 adapted to extendthrough any sludge at the bottom of the tank and rest on the bottom wall16 of the tank 10.

The base assembly 42 also includes a skirt or flange 64, generallyfrustoconical in shape, which flares upwardly and radially outwardlyfrom the annular ring member 44 and which extends in a verticaldirection above four port openings 71-74 (FIGS. 1 and 3) in the lowerend 40 of the cylindrical casing 38.

As best shown in FIGS. 2 and 3, the sensor assembly 32 extendsdownwardly through the open upper end 36 of the cylindrical casing 38and is fixed at its upper end to a cap member 76 which includes acircular flat top portion 78 having at least one vent hole 79 (FIG. 3)and an annular portion 80 slightly larger in diameter than the diameterof the cylindrical casing 38. The cap member 76 is fixed to thecylindrical casing 38 by wing nuts 81-83 (FIG. 4).

Mounted to the top portion 78 of the cap member 76 is a generallyinverted U-shaped bracket member 86 having two upstanding leg portions87 and 88 and an upper bight portion 89 to which is affixed an eyelet 90which in turn has a cable 92 (FIG. 1) connected thereto for raising andlowering the leak detector 14 out of and into the tank 10 of petroleumproduct (or liquid HC).

The top portion 78 also has an opening therethrough for receiving atubular connector 94 which receives therethrough a multiconductor cable96 which, at its lower end 98 has a plurality 100 of insulated wireconductors (typically 25) extending therefrom which extend to and areconnected into the sensor assembly 32. The tubular connector 94 is ofthe type which will permit movement of the cable upwardly or downwardlyinto the cylindrical casing 38.

The sensor assembly 32 is mounted on a threaded rod 104 having a freelower end 106 and an upper end 108 which is journaled in the top portion78 of the cap member 76 and which has a knob 110 at the upper endthereof for rotating the rod 104.

Also fixed to and extending downwardly from the top portion 78 arespaced apart guide rods 111 and 112 which extend on either side of thethreaded rod 104 such that the two guide rods 111 and 112 (and thethreaded rod 104 therebetween) lie generally in the same plane. Theguide rods 111 and 112 are each fixed at their upper end to the topportion 78 of the cap member 76 and are each fixed at their lower endsto a circular plate 113 located adjacent a circular plate 114 of thebase assembly 42.

A hollow space 115 is defined between the plates 46 and 114 forreceiving a ballast material 116 such as lead shot or ball bearings.

Spaced slightly above the lower circular plate 113 is a guide block 118.The lower end of the threaded rod 104 is journaled in the guide block118 which has two spaced apart openings therein for enabling the block118 to be slidably received on the guide rods 111 and 112.

The sensor assembly 32 includes an upper block 120 which has twoopenings for slidably receiving the guide rods 111 and 112. It will beunderstood from FIGS. 2, 5 and 6 that the block 120 also has a centralthreaded opening through which the threaded rod 104 extends wherebyrotation of the knob 110 to rotate the threaded rod 104 will causeraising or lowering of the block 120 on the guide rods 111 and 112.

The sensor assembly 32 further includes a cable connector block assembly122 which is mounted on top of the upper block 120. Also mounted to theupper block 120 and extending downwardly therefrom is a sensor plateassembly 124 into which the multiple insulated conductors 100 extend andin which they are clamped in an equally spaced generally verticalparallel manner as shown in FIGS. 2, 3, 7 and 8. The lower edge of theplate assembly 124 has a bottom edge 126 which is inclined to thehorizontal at a predetermined angle θ as shown in FIG. 7. The insulatedwire conductors 100 are insulated from metallic parts of the plateassembly 124 which can be made of metallic conducting and/or nonmetallicnonconducting plate portions so long as the insulated wire conductors100 are insulated from one another and from an outer metal plate 128 ofthe plate assembly 124 which serves as a ground (current return)element.

However, the lower ends of the wire conductors 100 such as wireconductor ends 201, 202, 203, 204 . . . 219, 220, 221, 222, 223, and 224are bare, uninsulated, exposed end portions 201-224 spaced verticallyabove each other a predetermined distance H and horizontally apredetermined distance L from each other on the inclined bottom edge 126of the plate assembly 124 as shown in FIG. 7 for providing an array ofsensors (conductor end portions) 201-224 having a predeterminedsensitivity as will be described in greater detail hereinafter.

The circular plates 46 and 114 are held to the circular plate 113 by abolt 230 which extends through the center of the assembly 42.

Also mounted to the top portion 78 of the cap member 76 is a fitting 232which receives a tubing 233 that extends through the cap member 76 to acoupling 234 (FIG. 6) mounted to the block 120 and connected to a tubingmember 235 having a curved end portion 236 that is positioned so that anopen end 238 thereof is aimed at and in line with the area adjacent toand parallel to the lower edge 126 of the sensor plate assembly 124. Thetubing 233 is flexible and extends through the fitting 232 to thecoupling 234 connected to the tubular member 235 to enable an operatorto wash, scrub or flush off the array of sensors (bare conductor endportions) 201-224 periodically such as with the petroleum liquid in thetank 10.

In the use of the leak detector 14 (after a quantity of water isintroduced into the tank 10 to cover the bottom 16), an operator willfirst lower the leak detector 14 through the hole 12 as shown in FIG. 1.The operator then will let the detector 14 settle through any sludge atthe bottom 16 of the tank 10 until it is safe to assume that thedetector 14 is resting on the bottom 16 of the tank 10. The detector 14is allowed to rest in this position as shown in FIG. 1 for a shortperiod of time after which it is raised out of the tank 10. A generallyvertical layer of water sensing material 248 is provided on the exteriorsurface of the casing 38 as shown in FIG. 1. The portion 249 of thelayer 248 situated in the water will change color to establish a waterlevel line 250. Alternatively, a float type or optical sensor can beused to initially determine the water level line 250.

After the detector 14 is raised out of the tank 10, the operator willturn the knob 110 to adjust the position of the sensor assembly 32 sothat the inclined lower edge 126 of the plate assembly 124 having thesensors (conductor end portions) 201-224 thereon will extend through thehorizontal plane containing the water level line 250 and interface 24between the water layer 20 and the liquid hydrocarbon 22 in the tank 10when the detector 14 is reinserted into the tank 10.

Then the operator again lowers the leak detector 14 back into the tank10 to the bottom 16 thereof so that the leak detector 14 is restingfirmly on the bottom 16 of the tank 10. Then conductivity can bedetected using a conventional conductivity meter, ohm meter or voltmeter.

In one embodiment shown in FIG. 9, a display panel 254 can be utilizedwhich will indicate the position of the interface relative to theinclined lower edge 126 of the plate assembly 124. This is accomplishedby the electrical circuits 30 shown in FIG. 10. Each circuit 30 includesone of the conductors 100 and the sensor end portion thereof, such asthe conductor end portion 201. Each circuit 30 also includes the liquidbetween the sensor (e. g., sensor 201) and the grounded plate 128, acommon conductor 256 connected to the plate 128, a lamp circuit 258including a lamp (LED) 260, a resistor 262 and a capacitor 264, and asignal processing circuit 266 including four resistors 271-274, a NANDgate 276 and an inverter 278.

In addition or as an alternative, the sensors or conductor ends 201-224can be coupled to input ports of a microprocessor 280 (FIG. 9) whichwill note the conductivity or voltage levels on each of the conductorsconnected to each of the sensors (conductor ends) 201-224 and note thevoltage levels at the respective input ports and what changes take placeover a given period of time.

Another conductivity measuring alternative is a multi-position switchwith an AC conductivity meter.

In this way, one can determine over a period of time whether there hasbeen an increase in moisture or water in the tank 10 or whether therehas been a leak by reason of a decrease of water in the tank 10.

Each of the circuits 30 provides a simple indication of the status ofthe water-petroleum liquid interface 24 and can be used in addition toor in place of a conductivity meter 282 on the control panel 254 shownin FIG. 9. The position of the interface layer 24 is indicated by a rowor column 286 of twenty-four (24) light emitting diodes lamps 260, eachone corresponding to and coupled to one of the sensor forming conductorend portions 201-224.

The LEDs 260 corresponding to the sensors immersed in the lowconductivity hydrocarbon liquid 22 will flash on and off approximatelytwice per second. Those LEDs 260 associated with the water-contactingsensors 201-224 remain continuously energized. In this way, if the levelof the interface 24 changes due to a leak, those ones of the LEDs 260that were previously energized begin to flash on and off as the waterlevel 18 in the tank 10 decreases.

Each of the circuits 30 can be energized by batteries so that the leakdetector has a self-contained power supply.

As noted above, FIG. 9 illustrates a leak detector system 300 includingthe leak detector 14, the control panel 254 and the optionalmicroprocessor 280. The control panel is shown with two rows of sixteen(16) lamps, although as noted above preferably one row or column oftwenty-four (24) lamps are preferred. Also, the control panel 254 isshown with the conductivity meter 282 mounted thereon together with acalibration control knob 302. These items can be eliminated, if desired.Also an ON and OFF switch 304 can be provided for energizing the controlpanel 254.

Further, it will be understood that all monitoring can be effectedsolely with the microprocessor which can be programmed to poll theinputs thereof connected to the sensors 201-224 periodically, such asonce every hour. Then, of course, any changes in conductivity over anynumber of the sensors or bare end portions 201-224 can be correlatedwith the distance H, the distance L, the angle θ and the time elapsed todetermine the rate of leakage.

It should be noted that the sensitivity of the array of sensors can bealtered by changing the angle θ, the angle of inclination of the loweredge 126 of the plate assembly 124 to the horizontal.

From empirical tests, it appears that a preferred angle for the angle θis 20°.

In FIG. 8 there is illustrated a modified embodiment of a sensorassembly 322 including a plate assembly 324 wherein the plate assembly324 is adapted to be rotated about an axis perpendicular to the plateassembly 324 so as to alter the angle β. In this assembly, a lower edge326 of the plate assembly 324 is perpendicular to the side edges of theplate assembly 324. In this embodiment, the sensors 330 are thermalsensors each of which extends an equal distance from the edge 326. Thesensitivity of this array of sensors is adjusted by rotating the plateassembly 324 so as to change the angle of the lower edge 326 relative tothe horizontal. This will result in a change in the distance H measuredvertically between the exposed ends of adjacent sensors of the plurality330 of sensors.

In a thermal system, the same voltage is applied to each sensor 330 andthe liquid medium will determine how much heat is dissipated by eachsensor and the rate of heat dissipation. This will determine a givensteady state current. Then when the liquid medium is changed, such asfrom petroleum product to water, the heat dissipation will changecausing a change in the steady state current. This change in current ismeasured to determine a change in the water level.

Preferably, the exposed end portions 201-224 of the sensing conductors100 are "tinned", i.e., coated with an anti-corrosive metal, such assilver, gold or tin.

In FIG. 11 there is illustrated an alternative embodiment of a leakdetector assembly 414 of the present invention where, instead of havinga rotatable plate assembly 324 as shown in FIG. 8 which lies in agenerally vertical plane and is rotated in that plane, the modified leakdetector 414 shown in FIG. 11 includes a plate assembly 426 which alsolies in a vertical plane but which is rotatable about an axis lying inor parallel to the plane of the plate assembly 424. Here a plurality 430of insulated wire conductors are mounted on one side of the plateassembly 424 and when the plate assembly 424 is rotated, the relativevertical distance between adjacent bare end portions of the insulatedwire conductors 430 is changed in a manner similar to rotating the plateassembly 324 in FIG. 8 as described above.

It will be appreciated that in the embodiment shown in FIG. 7, thesensitivity is fixed whereas in the embodiment shown in FIGS. 8 and 11,the sensitivity of the array of sensors can be altered.

Further it will be appreciated that the frustoconical skirt or flange 64provides a deflecting baffle for deflecting any debris at the interface24 between the liquid 22 and the water 20 away from the ports 71-74 whenthe leak detector 14 is lowered into the tank 10. Additionally, in caseany of this debris should gain access to the interior of the cylindricalcasing 38, the tubing member 235 for washing or flushing the inclinedlower edge 126 of the plate assembly 124 enables the sensors 201-224 tobe washed or cleaned.

Extensive testing of the leak detector was conducted at an Amocofacility in Casper, Wyo. and produced surprisingly good results.

For example, leak tests for a tank with gasoline (tank No. 325) and atank for light fuel oil (tank No. 322) maximum leak rates of 15 gal./hrand 10 gal./hr were detected using a leak detector constructed accordingto the above teachings. Accuracy of the detector is verified bydetermining the leak rate while drawing water from the tank using thedetector and comparing that determined rate with the actual measuredrate of water drained off from the tank.

From the foregoing description, it will be apparent that the method ofthe present invention using the leak detector 14, 414, has a number ofadvantages, some of which have been described above and others of whichare inherent in the invention.

Also it will be apparent from the foregoing description that variousmodifications can be made to the method of the present invention withoutdeparting from the teachings of the invention. Accordingly, the scope ofthe invention is only to be limited as necessitated by the accompanyingclaims.

We claim:
 1. A method for determining the presence of a leak in an aboveground tank of liquid hydrocarbons having a lower specific gravity and alower conductivity than water, said method comprising the stepsof:determining the level of water in said tank; sensing changes in thelevel of water in the tank over time by sensing changes in conductivityat points along a line extending across an interface between the liquidhydrocarbons and the water in the tank; and, determining the amount andrate of leakage relative to the capacity of the tank and the change inthe level of the water in the tank over a selected time period.
 2. Amethod for determining the presence of a leak in an above ground tank ofliquid hydrocarbon having a lower specific gravity and a lowerconductivity than water, said method comprising the steps of:determiningthe level of water in the tank; sensing changes in the level of water inthe tank including the step of providing an array of sensors along aline inclined to the horizontal and placing said line of sensors acrossthe interface between the liquid and the water in the tank; and,determining the amount and rate of leakage relative to the capacity ofthe tank and the change in the level of the water in the tank over aselected time period.
 3. The method of claim 2 wherein said step ofsensing the changes in the level of water in the tank includes detectingwhich sensors are in the liquid and which sensors are in the water andthe change in the number of sensors in the liquid and the change in thenumber of sensors in the water over a preselected time period.
 4. Themethod of claim 2 wherein said step of sensing changes in the level ofthe water in the tank includes sensing changes in the conductivity ineach one of a plurality of electrical circuits that each include one ofsaid sensors.
 5. A method for determining the presence of a leak in anabove ground tank of liquid hydrocarbon having a lower specific gravityand a lower conductivity than water, said method comprising the stepsof:determining the level of water in the tank; sensing changes in thelevel of water in the tank over time including providing an array ofsensors along a line inclined to the horizontal and placing said line ofsensors across an interface between the liquid and the water in thetank; periodically washing or flushing the array of sensors; and,determining the amount and rate of leakage relative to the capacity ofthe tank and the change in the level of the water in the tank over aselected time period.
 6. A method for determining the presence of a leakin an above ground tank of liquid hydrocarbons having a lower specificgravity and a lower conductivity than water, said method comprising thesteps of:determining the level of water in said tank including providinga generally vertical layer of water sensitive material on the outersurface of a casing; lowering said casing having an array of sensorstherein into said tank and allowing said casing to rest on the bottom ofthe tank for a short period of time; raising the casing out of the tank;and noting where a water level line is established by the change incolor of that portion of the layer which was situated in the water;sensing changes in the level of water in the tank over time; and,determining the amount and rate of leakage relative to the capacity ofthe tank and the change in the level of the water in the tank over aselected time period.
 7. The method of claim 6 wherein said step ofsensing a change in the level of the water in the tank includesadjusting the position of the array of sensors so as to be on a lineinclined to the horizontal and intersecting the plane of the interfaceas indicated by the water level line on the outside of the casing.
 8. Amethod for determining the presence of a leak in an above ground tank ofliquid hydrocarbons having a lower specific gravity and a lowerconductivity than water, said method comprising the steps of:determiningthe level of water in said tank; sensing changes in the level of waterin the tank by sensing changes in conductivity at points along a lineextending across an interface between the liquid hydrocarbons and thewater in the tank including monitoring the conductivity at discrete,spaced, vertical positions in the liquid just above the interface ofliquid and water and in the water just below the interface and notingchanges in conductivity at those positions over time; and, determiningthe amount and rate of leakage relative to the capacity of the tank andthe change in the level of the water in the tank over a selected timeperiod.
 9. A method for determining the presence of a leak in an aboveground tank of liquid hydrocarbons having a lower specific gravity and alower conductivity than water, said method comprising the stepsof:determining the level of water in said tank; sensing changes in thelevel of water in the tank by sensing changes in conductivity at pointsalong a line extending across an interface between the liquidhydrocarbons and the water in the tank including monitoring the thermalconductivity (heat dissipation) at discrete, spaced, vertical positionsin the liquid just above the interface of liquid and water and in thewater just below the interface and noting changes in conductivity atthose positions over time; and, determining the amount and rate ofleakage relative to the capacity of the tank and the change in the levelof the water in the tank over a selected time period.
 10. A method fordetermining the presence of a leak in an above ground tank of liquidhydrocarbons having a lower specific gravity and a lower conductivitythan water, said method comprising the steps of:introducing an amount ofwater into the tank sufficient to cover the bottom of the tanksufficient to enable a determination of water level in the tank to bemade; determining the level of water in said tank; sensing changes inthe level of water in the tank over time; and, determining the amountand rate of leakage relative to the capacity of the tank and the changein the level of the water in the tank over a selected time period.
 11. Amethod for determining the presence of a leak in an above ground tank ofliquid hydrocarbons having a lower specific gravity and a lowerconductivity than water, said method comprising the steps of:determiningthe level of water in said tank; sensing changes in the level of waterin the tank over time; determining the amount and rate of leakagerelative to the capacity of the tank and the change in the level of thewater in the tank over a selected time period; drawing water from tankat a known rate; and, repeating the above step to verify the leak ratedeterminations made.
 12. The method of claim 11 further including thestep of using the results of the verification to calibrate a leakdetector used to make the leak rate determination.